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A new approach for developing highly sensitive MOS photodetector based on the assistance of anodic aluminum oxide (AAO) membrane is proposed, fabricated, and characterized. It enables the photodetector with the tunability of not only the intensity but also the range of the response. Under a forward bias, the response of the MOS photodetector with AAO membrane covers the visible as well as infrared spectrum; however, under a reverse bias, the near-infrared light around Si band edge dominates the photoresponse. Unlike general MOS photodetectors which only work under a reverse bias, our MOS photodetectors can work even under a forward bias, and the responsivity at the optical communication wavelength of 850nm can reach up to 0.24 A/W with an external quantum efficiency (EQE) of 35%. Moreover, the response shows a large enhancement factor of 10 times at 1050 nm under a reverse bias of 0.5V comparing with the device without AAO membrane. The underlying mechanism for the novel properties of the newly designed device has been proposed.
©2010 Optical Society of America
1. Introduction
In recent years, many semiconductor photodetectors have been fabricated and studied, such as heterojunction and p-i-n homojunction photodiodes [1–5]. For the purposes of the integration of photodetectors with electronics, Si-based photodetectors provide the most outstanding alternative for monolithic integration with optoelectronic applications using a CMOS process. However, due to its indirect bandgap, Si has a larger absorption length and a lower absorption coefficient. Therefore, many efforts have been made for increasing the effective absorption length and enhancing quantum efficiency of specific wavelength, such as resonant-cavity photodetectors [6–9]. Among them, Si NMOS and PMOS photodetectors have received a great deal of attention because of their mature technology and low cost [10–12]. To fabricate the photodetectors, well controlled rapid thermal oxidation (RTO) technique has been used to deposit the ultrathin oxide layer, which has an excellent quality with very few defects. Thus, the dark current can be reduced down to the order of 10−9 ampere. Nevertheless, due to the defect-free SiO2 layer, the working voltage is restricted in the reverse bias range [10–12]. In this paper, MOS photodetectors with a pre-determined anodic aluminum oxide (AAO) nanostructure are proposed, fabricated, and characterized. It is found that this new MOS photodetectors can work under both forward and reverse bias voltages. Besides, the working wavelength is tunable within a wide spectrum spanning from visible to infrared range. The underlying mechanism responsible for the unique characteristics of this new photodetector has been proposed. It is believed our approach presented here should be very useful for the future development of highly sensitive photodetectors.
2. Experiment
In this experiment, the gate oxide of the MOS photodetector was grown for 45 seconds by rapid thermal annealing (RTA) on 5-10 Ω-cm n-type (100) Si substrate at 900 °C in nitrogen and oxygen ambient (flow rate ratio 1:4). Before oxidation, the Si wafer was washed with deionized water and dipped in HF solution. The thickness of SiO2 measured by ellipsometry was 5 nm.
The as-prepared Si substrate was coated with a 300 nm aluminum layer by thermal evaporation method. During the preparation of AAO, anodization was conducted at 40 V dc in 0.3 M oxalic acid at 0 °C in a constant-temperature bath. In the process, only the central area of aluminum contacting with oxalic acid became alumina. The other part of aluminum was used as the front electrode. Then the sample was dipped in the 5% phosphoric acid for five minutes in order to expand the pores and remove the alumina barrier layer [13]. The other side of the sample was coated with an aluminum back contact. The forward bias is defined by the terminology of conventional diodes. A schematic view of the sample structure is shown in Fig. 1(a) .
Fig. 1 (a) Schematic shows the cross sectional view of the MOS photodetector with AAO nanostructure. (b) Scanning electron microscope (SEM) image of the top view of AAO nanostructure.
For the reflection spectrum measurement, a Xenon lamp and Spectra Pro 300i monochromator were used as the light source. The reflection spectra were detected by a silicon detector. For the responsivity measurement, our system uses a wide-spectrum light source. The white light is chopped and diffracted into separated monochromatic narrow bands, and each of which is projected onto the AAO region on the MOS photodetector. The photocurrents generated by incident monochromatic light are converted and amplified to an ac voltage signal by a trans-impedance amplifier. Then, a lock-in amplifier is utilized to measure the ac voltage signal which has the same modulation frequency as the chopper frequency (140Hz). The light intensity of each wavelength is calibrated by a NIST-traceable Si detector. So the responsivity (R) of the MOS photodetector can be calculated. External quantum efficiency (EQE) can be evaluated by using the equation:
The morphology of AAO was characterized by scanning electron microscopy (SEM) (JSM 6500, JEOL) as shown in Fig. 1(b). The pore size of AAO was estimated in the range of 30-50 nm. The standard high-frequency capacitance-voltage (C-V) measurement of the MOS photodetector was performed by an HP4284 precision LCR meter.3. Results and discussion
Before entering to investigate the photoelectric properties of our designed device, we first measure the reflection spectrum of AAO membrane since it may influence the absorption spectrum of the underlying device. As shown in Fig. 2 , there exist two valleys at around 500 nm and 800 nm, which can be attributed to the effect of AAO structure. The maximum or minimum reflectance of the AAO membrane follows the equation:
where n is the refractive index of the AAO membrane, d is the AAO thickness, γ is the refraction angle, m is the order of interference, and λ is the wavelength at maximum or minimum reflectance [14]. The m values corresponding to two valleys at around 500 nm and 800 nm are 5/2 and 3/2, respectively. In our calculation, d is 360 nm, γ is 0°, and then n can be estimated as a value of 1.67, which is in the range reported in the reference [15]. For comparison, the reflection spectrum of pure Si surface shows a monotonic decrease from visible to infrared range.Figure 3 shows the current-voltage (I-V) curves of the MOS photodetector with and without an incident light beam. It shows that the dark current under a forward bias is lower than that under a reverse bias. It is contrary to the general case, in which the dark current is larger under forward bias than under reverse bias, because the dark current is dominated by majority electrons under forward bias and by minority holes under reverse bias. But if there are defects in SiO2, the situation can be different. Under a forward bias, the accumulated electrons are trapped by lower defect levels. In addition to the quantum tunneling, in order to contribute to the conduction, the trapped electrons need to overcome the potential barrier between the lower defect level and the aluminum Fermi level as shown in Fig. 6(a) . Therefore, the dark current under forward bias is reduced. However, under a reverse bias, minority holes not only can directly tunnel through SiO2 layer, but also can transport through the defect levels in SiO2 as shown in Fig. 6(c). The dark current can then be enhanced under reverse bias. Therefore, the result of the dark current reveals the existence of defect levels in SiO2, which provide additional conducting channels for carriers to flow through [16–20]. These results are quite different from those of the MOS photodetector reported by other authors [10–12], in which the devices can only work well under a reverse bias voltage. The I-V characteristics indicate that the photocurrents occur not only under a reverse bias but also a forward bias. It is also found that the measured photocurrent is not zero when the applied bias is zero. This is because the incident light causes an upward shift of the Fermi level of silicon, which is higher than the Fermi level of aluminum contact. Therefore, electrons are inclined to accumulate at the SiO2-Si interface. The phenomenon can also be explained by the mechanism as shown in Fig. 6(a).
Fig. 3 Current-voltage (I-V) characteristics of the MOS photodetector with AAO membrane under no illumination and under a Xeon lamp.
Fig. 6 The schematic energy band diagrams of the MOS photodetector with AAO membrane under (a) forward, (b) reverse and (c) higher reverse bias.
Figure 4 shows the responsivity of the MOS photodetector under various bias voltages. As we can see, the response covers the whole visible spectral range and a part of near-infrared region. The two pronounced peaks in Fig. 4(a) correspond to the reflection valleys of AAO nanostructure as shown in Fig. 2. The spectra also show that the response in the visible range and a part of near-infrared range (800 nm – 1000nm) is much higher than that near Si band edge. In addition, under a forward bias voltage, higher voltage causes the higher response both in the visible and near-infrared region. When the applied bias is reversed, in contrast to that of the case of forward bias, the near Si band edge signal is much larger than that of visible region as shown in Fig. 4(b). The relative ratio of the responsivity between infrared and visible region increases with increasing the magnitude of reversed bias. For comparison, the photoresponse of the photodetector without AAO membrane has been performed. The responsivity shown in Fig. 4(c) exhibits quite different results. Unlike the dominance of 400 nm - 1000 nm response as shown in Fig. 4(a), Si band-edge response completely over-rules the whole spectra for both forward and reverse bias.
Fig. 4 (a) and (b) show the measured responsivity of the MOS photodetector with AAO nanostructure under forward and reverse bias, respectively. (c) shows the measured responsivity of the same device with AAO membrane being removed under the bias of −0.5 V and 0.4 V.
In order to have a better understanding of the working condition of our photodetector under different bias, we have performed C-V measurement. The C-V curve shown in Fig. 5 demonstrates that the photodetector works in the depletion condition under a reverse bias and in the accumulation condition under a forward bias under working voltages from −0.5V to 0.4V. Moreover, under a higher reverse bias, the photodetector can be driven into the deep depletion condition due to the broadening of the depletion width. The responsivity of the photodetector under the condition is not shown in this report.
Here, we propose a possible underlying mechanism to describe the observed behavior. As shown above, the SiO2 layer contains several defects responsible for the transitions in the visible range. It has also been reported that there exist two groups of defect levels in SiO2, which can be used to explain the visible electroluminescence from MOS structure [17–20]. The existence of defect levels may provide alternate paths for electrons and holes to pass through. Figure 6(a) shows the band diagram of the MOS photodetector operating under a forward bias voltage. There are two possible paths contributing the measured photocurrents. First, the accumulated electrons in the Si conduction band near the SiO2 and Si interface may be trapped by lower defect levels, and then pumped into higher defect levels by absorbing visible light. Hereafter, the electrons travel from the higher defect levels to the Fermi level of aluminum contact.
The second possibility contains the process that the electrons in the Si valence band absorb the incident light beam and jump to the conduction band. The photoexcited electrons travel through the higher defect levels in SiO2 or directly tunnel through 5 nm SiO2 and then enter the Fermi level of aluminum. Due to the band filling effect, the peak position is centered at around 1050 nm rather than 1150 nm, the wavelength of the Si band edge emission. The wide range of the responsivity covering visible and infrared radiation therefore can be understood.
Figure 6(b) and 6(c) show the MOS photodetector operating under reverse and higher reverse bias, respectively. We can see that the visible response gradually decreases when the aluminum Fermi level exceeds the lowest level of the higher defect level group in SiO2. The electrons traveling from aluminum are directly trapped by the higher defect level group rather than the lower one. Therefore, in SiO2, with increasing the reverse bias, fewer and fewer electrons can be trapped by the lower defect levels and contribute to the absorption of visible light. Consequently, higher reverse bias will result in lower visible and higher near-infrared response.
Finally, we interpret the striking behavior that the response of the photodetector without AAO membrane is much lower than that of the photodetector with AAO membrane, although AAO reflects more light than pure Si. This interesting result may be due to the fact that the remaining aluminum [21] in AAO membrane provides a much better conducting path for the photoexcited carriers than that of the device without AAO membrane. Therefore, the AAO membrane with its underlying Al provides an excellent nanoscale conducting network to assist the transport of photoexcited carrier to the electrodes without being grabbed by trapping center. The occurrence of two peaks at Si band edge as shown in Fig. 4(c) may arise from the existence of the quantized level in the triangular well near the SiO2-Si interface [22]. It is stressed here that unlike general MOS photodetectors which only work under a reverse bias, our MOS photodetectors can work even under a forward bias, and the responsivity at the optical communication wavelength of 850nm can reach up to 0.24 A/W with an external quantum efficiency (EQE) of 35% based on the calculation using Eq. (1).
4. Conclusions
In summary, a tunable and wide range MOS photodetector covering visible and infrared radiation has been demonstrated based on the assistance of AAO membrane. Notably, the photoresponse can be enhanced by as large as 10 times, under −0.5 V bias voltage, at 1050 nm. Furthermore, the response at the short haul optical communication wavelength of 850 nm can be achieved up to 0.24 A/W with an external quantum efficiency of ~35% with the working voltage of 0.4 V. Therefore, our result shown here may pave a new route for the development of highly sensitive photodetectors.
Acknowledgments
This work was supported by the National Science Council and Ministry of Education of the Republic of China. We like to thank Professor J. G. Hwu for the assistance of the C-V measurement.
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